1. Field of the Invention
The present invention relates to technologies of a current output circuit and a DA converter circuit, and more particularly to a display device and an electronic apparatus mounted with the current output circuit or the DA converter circuit.
2. Description of the Related Art
In recent years, demand for a thin display device displaying images has been increasing. As the thin display device, a liquid crystal display device displaying images by using a liquid crystal element is widely used in various types of display devices, such as portable telephones and personal computers by utilizing the advantages of the liquid crystal display device such as thin size, high image quality, and light weight.
On the other hand, the development of a thin display device and a light emitting display device using a light emitting element has also been advancing. Such a light emitting element includes various kinds of elements over a wide range, such as an organic material, an inorganic material, a thin film material, a bulk material, and a dispersion material.
An organic light emitting diode (OLED) is a typical light emitting element currently seen as promising for all types of thin display devices. An OLED display device using an OLED element is thinner and lighter than the existing liquid crystal display devices, and in addition, have characteristics such as a high response speed suitable for a moving image display, a wide viewing angle, and a low voltage drive. Therefore, the OLED display device is drawing attention as the next-generation display device since a wide variety of its applications are anticipated, to portable telephones, portable information terminals such as a personal digital assistant (PDA), televisions, monitors, and the like.
In particular, an active matrix (AM) OLED display device realizes a large screen display and high definition which are difficult for a passive matrix (PM) display. Furthermore, the AM-OLED display device operates at lower power consumption than the PM-OLED display device, and has high reliability. Thus, it is strongly expected to be put into practical use. Also, by integrating driver circuits on a panel, a frame region of the panel can be narrowed, thus a display device with a high added value can be obtained. This is another advantage of the AM-OLED display device.
An OLED element is a current drive type element which is structured by an anode, a cathode, and an organic compound containing a layer sandwiched between the anode and the cathode. The brightness of light emitted from the OLED element is roughly proportional to the amount of electric current flowing in the OLED element.
A voltage programming method and a current programming method are used as driving methods for displaying images in AM-OLED display devices. The voltage programming method is a method in which a video signal of voltage value data is inputted to pixels as an input video signal. On the other hand, the current programming method is a method in which a video signal of current value data is inputted to pixels as an input video signal. Generally, in the AM-OLED display devices, the current programming method tends to be preferably used.
The current programming method is preferably used in the light of the display quality. In a pixel of the AM-OLED display device, a pixel drive transistor controlling brightness of light emitted from an OLED element of the pixel is connected in series with the OLED element in both voltage and current programming methods. In the voltage programming method, a voltage of a video signal is normally applied directly to a gate electrode of a pixel drive transistor. Therefore, if there is variation, not uniformity, in the electrical characteristics of the pixel drive transistors across each of the pixels when the OLED elements emit light at a constant current, (then) the variation will develop in the current for driving the OLED element of each of the pixels. Variation in the current for driving the OLED element becomes variation in the brightness of light emitted from the OLED element. Further, variation in the brightness of light emitted by the OLED element reduces the quality of the displayed image as a sandstorm state or carpet-like pattern unevenness is seen over an entire screen.
In particular, polycrystalline silicon (polysilicon) TFTs are used as the pixel drive transistors at present for obtaining a sufficient current required for high brightness, which can not be obtained by using amorphous silicon thin film transistors (TFTs) as the pixel drive transistors. However, there is a problem with polysilicon TFTs in that variation in the TFT electrical characteristics are likely to develop due to faults in the crystal grain boundaries and the like.
Although the current programming method is suited for the AM-OLED display device than the voltage programming method in general, it has problems. One of the problems is that the configuration of its driver circuit is comparatively complicated than that of the voltage programming type, thus is more difficult to be integrated on a panel.